Precipitation behavior in Ag-modified L1 2–Al 3Ti and L1 0–TiAl(Ag)

Abstract

A detailed transmission electron microscopy (TEM) study has been made of the crystallographic characteristics of precipitation of L1 0–TiAl and D0 23–Al 11Ti 5 in Ag-modified L1 2–Al 3Ti as well as the precipitation of L1 2–Al 3Ti in L1 0–TiAl(Ag) matrix in Ti–Al–Ag ternary system. TEM observations revealed that L1 0–TiAl precipitates form a kinked plate surrounded by four {225} planes in L1 2 matrix. D0 23–Al 11Ti 5 precipitates form multi-domain structures which can be considered as twin related structures with the tetragonal axis of twin I parallel to [100] direction and that of twin II parallel to [010] direction of the L1 2 matrix. On comparison plate-like L1 2–Al 3Ti precipitates form a kinked plate surrounded by four {112} planes in L1 0 matrix. The particular habit plane, orientation relationship and twin plane variants were carefully determined. It is found that these crystallographic and morphological features can be explained excellently by adopting the linear elastic theory developed primarily by Khachaturyan. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.